Nodularia sp nifH gene transcripts in the Baltic Sea proper

Nitrogen fixation contributes significantly to the nitrogen input to the Baltic Sea. To develop a better understanding of the dynamics of plankton productivity, it is important to identify the N-2-fixing organisms and to determine their activity and distribution. In the Baltic Sea proper, we identified two cyanobacteria, Nodularia and Aphanizomenon, as the main nitrogenase-transcribing organisms in surface waters (4 m) based upon a library of 84 nitrogenase gene (nifH) clones. Using a quantitative species-specific real-time PCR protocol, Nodularia nifH gene transcription was found to be temporally and spatially variable with a strong peak at early to mid day and large variations at a single station over periods of months. The highest Nodularia nifH transcript concentration was observed in July (3.4 x 10(6) nifH copies L-1). Large variations were observed between stations, presumably due to patchiness of Nodularia biomass and variations in the exact day time of sampling. Rapid quantification of nitrogenase transcripts at the species level represents an important advancement for research concerning the distribution and regulation of planktonic cyanobacterial N-2-fixation.